A prominent feature of AIDS is the heterogeneity among HIV-1 retroviral genomes. Molecular cloning, restriction enzyme analysis, nucleotide sequencing, and infectivity studies have been used to characterized the various isolates. These studies provide information regarding their structural as well as geographic diversity and, possibly, the degree of antigenic variability and pathogenicity among different HIV-1 isolates. Virus prepared from the different molecular cloned HIV isolates are presently being used to prepare monoclonal antibodies to test their neutralization activity against a wide spectrum of HIV isolates. A T cell clone (ACH-2) from lymphocytes infected with HIV-1 (LAV) produced HIV-1 in response to stimulation with phorbol myristic acetate (PMA). An isolate was molecularly cloned and, upon transfection into T cells, produced virus; however, only in the presence of PMA. Passage onto fresh lymphocytes produced virus as monitored by reverse transcriptase activity (RT) in the absence of the inducing PMA. Mixing and matching experiments swapping the gag-pol-env region between the ACH-2 clone and LAV parent clone, restored biological activity of ACH-2 to wild-type LAV. The ACH-2 isolate may provide an understanding of the mechanism of pathogenicity of HM as it relates to chronic infections and/or latency and viral induction.